Solder flow stops for semiconductor die substrates

ABSTRACT

A substrate, which has semiconductor die arranged thereon, uses at least one solder flow stop, closely surrounding at least a portion of at least one mounting pad on which the die are mounted, to prevent die rotation during solder reflow. The at least one solder stop is non-wetting, during a solder reflow process, to solder used to mount the die.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims benefit of U.S. ProvisionalApplication No. 60/564,664, filed on Apr. 21, 2004, entitled INSULATEDMETAL SUBSTRATE USING SOLDER FLOW STOPS, to which a claim of priority ishereby made and the disclosure of which is incorporated by referenceherein.

FIELD OF THE INVENTION

The field of the invention is the mounting of semiconductor die onconductive pads of a substrate and preventing the rotation of the dieduring solder reflow.

BACKGROUND OF THE INVENTION

Semiconductor die are commonly soldered or otherwise secured to theconductive pads of a substrate. One typical substrate is an insulatedmetal substrate (IMS).

Thus, IMS substrates are well known for mounting electronic componentsand insulating them from the substrate on which the devices are mounted.IMS reduces thermal impedance and conducts heat more efficiently thanconventional printed circuit boards. While the invention is describedherein as applied to an IMS substrate, it will be understood that theinvention applies to a novel process of securing semiconductor die tothe die pads of any type of substrate including direct-bond copper(DBC), FR4 and the like.

FIG. 2 shows a cross section of a conventional IMS structure. Theconventional IMS structure includes a dielectric layer 14, which isusually a polymer sandwiched between copper circuit layer 12 andaluminum base layer 16. The circuit layer 12 will be patterned byconventional processes to provide pads for die mounting and relatedtraces which form wiring interconnects between die. The dielectric layer14 may be any conventional dielectric material that bonds to layers 12and 14, and is thin and thermally conductive. Once the pads for diemounting are defined on the circuit layer, semiconductor die are to besoldered thereto.

Automated placement and soldering of the die is performed by placing thedie with conventional pick and place tools together with solder wafers,and by a process of solder reflow to bond the die to the mounting padsdefined in the circuit layer 12. Thereafter, the die are wire bonded toone another and/or to conductive lands of the substrate on which otherdie are mounted.

It has been found that, during the conventional solder reflow process,the rectangular die rotate more or less about an axis perpendicular totheir surfaces. With the die so misaligned, the die or wire used to bondthe die are subject to damage during wire bonding. The die may bedamaged due to the bonder head striking improperly positioned die. Thewire may be damaged by the bonding tool and misplaced wire bond.

It would be very desirable to prevent such die rotation during thesolder reflow operation to prevent such die and wire damage during wirebonding.

SUMMARY OF THE INVENTION

In accordance with the invention, a novel solder flow stop is used toprevent die rotation of semiconductor die, on a conductive pad of asubstrate, during solder reflow. It is known to use solder stops, damsor barriers to control solder flow during reflow of the solder. Suchsolder stops are disclosed, for example, in U.S. Pat. No. 6,391,678 toPaszkiet et al.; U.S. Pat. No. 4,908,689 to McBride et al.; U.S. Pat.No. 5,282,565 to Melton; and U.S. Pat. No. 6,531,663 to Isenberg et al.In accordance with the invention, the solder flow stops surround themounting pads (or portions thereof) that receive the solder bondsbetween the die and the mounting pads. The die float on molten solderduring solder reflow, which can cause misalignment of the die in theabsence of the solder flow stops. A non-wetting solder flow stop acts torestrain the solder from flowing beyond the flow stop but furtherprevents misalignment (rotation) of the die.

Other features and advantages of the present invention will becomeapparent from the following description of the invention which refers tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one embodiment of the invention in a cross-sectionalview.

FIG. 2 illustrates a known insulated metal substrate in across-sectional view.

FIG. 3 show the results after solder reflow for soldering of componentson IMS using solder flow stops.

FIG. 3A is a cross-sectional view of a portion of FIG. 3, viewed atsection line 3A-3A on FIG. 3.

FIG. 4 illustrates another embodiment of the invention in across-sectional view.

DETAILED DESCRIPTION

FIG. 1 illustrates one embodiment of the present invention. A solderflow stop 11 surrounds a copper mounting pad 18, which is etched out ofcopper layer 12, on the surface of an insulated metal substrate 10. Flowstop 11 can be aluminum, which is non-wetting to a solder of tin-lead.Aluminum is also used for the flow stops 24, 26, 28, 30, 32, 34 in FIG.3; however, any material that is non-wetting to the solder and circuitlayer 12 during the solder reflow process may be used as the solder flowstop. Non-wetting means that the material is comparatively non-wettingwith respect to the material of circuit layer 12 (copper) and thesolder, such that the solder tends to stay on the mounting pad 18.Aluminum is a preferred material for this solder and circuit layer 12combination. Other solder flow stop materials may be preferred for othercombinations of solder and circuit layer 12 material.

Solder metals which can be used include tin-lead, tin-lead-silver,tin-antimony, lead-indium, tin-lead-indium, tin-bismuth,tin-lead-bismuth and the like.

The solder flow stop 11 may be substantially level with the pad 18 oflayer 12, as shown in FIG. 1. The gap 17 between the stop 11 and thecircuit layer 12 may have a very small finite gap distance or may beomitted entirely, abutting the circuit layer 12 directly against thestop 11. By providing a finite gap 17, thermal expansion mismatch andexcess solder may be accommodated. However, the gap should besufficiently small to prevent die rotation during reflow from exceedingabout 2 to 3 degrees. The solder flow stop 11 may have an upper surfacethat is elevated above the mounting pads 18 formed by the circuit layer12. Thus, as shown in FIG. 4, for example, the solder flow stop 11 isprinted on the circuit layer 12, directly, either before or afterpatterning of the circuit layer 12. In any case, the retention of thesolder on the mounting pad 18 helps to prevent misalignment of the dieduring solder reflow, such as by rotation of the die.

Referring next to FIG. 3, there is shown a suitable circuit substrate100, which may be IMS and has an aluminum baseplate 16 on which thecopper layer 12 has been patterned to form suitable conductive tracesand conductive mounting pads which are beneath semiconductor die 101 to109. In accordance with the invention, the mounting pads receiving die101 to 109 are surrounded, at least in part, by aluminum solder flowstop areas 24, 26, 28, 30, 32 and 34. Note that areas 26, 30, 32 and 34extend outward from their respective die to bond pad areas such as areas26 a, 30 a, 30 b, 32 a, 32 b, 34 a, 34 b. Other bond pad areas are alsoprovided as shown.

As shown in FIGS. 3 and 3A, after solder reflow, wire bonds can be madeamong the various die and bond pads, using a conventional ultrasonicwire bond process, to bond the bond wires, which may be aluminum, to thebond pads and the various die.

FIG. 3 shows the benefit of adding aluminum solder flow stop areas 24,26, 28, 30, 32, 34 on copper mounting pads. Molten solder does not wetthe aluminum flow stops of FIG. 3, which prevents the flow of solderfrom the mounting pads. Thus, die 101 to 109 remain aligned after solderreflow.

FIG. 3A is a cross-sectional view of a portion of FIG. 3 taken acrosssection line 3A-3A in FIG. 3. A baseplate 16 of aluminum is of 2 mmthickness. Dielectric layer 14 is a polymer dielectric material of 170μm thickness. Copper layer 12 is a circuit layer of 85 μm thickness. Thecopper mounting pad 18 is patterned within copper layer 12. The die 103is mounted on the copper mounting pad 18, and solder flow stops 28 ofaluminum, which is 40 μm thick, closely abut two edges of the die 103.An aluminum wire 44 is then wire bonded to the top electrode of die 103and to land 32 b, connecting the top of die 103 to the bottom of die105. Similar wire bonds are used throughout and will be well known tothose skilled in this field.

Significantly, each die is closely bounded by a solder flow stop whichalso blocks its rotation during solder reflow.

Although the above disclosure has focused on insulated metal substrates,it should be noted that the invention is not limited to insulated metalsubstrates, but is applicable to any substrates that use solder formounting one or more electronic components. Such substrates include, butare not limited to, epoxy-glass substrates, for example, FR4 substrates,paper phenolic substrates, DBC, ceramic substrates, silicon substrates,printed circuit boards (PCB), printed wiring bonds (PWB), and flexiblecircuits.

Although the present invention has been described in relation toparticular embodiments thereof, many other variations and modificationsand other uses will become apparent to those skilled in the art. It ispreferred, therefore, that the present invention be limited not by thespecific disclosure herein, but only by the appended claims.

1. A substrate having at least one conductive mounting pad on which atleast one die is mounted by soldering, said mounting pad having at leastone solder flow stop around at least a portion of its periphery andbeing relatively closely spaced from the periphery of said die.
 2. Thesubstrate of claim 1, wherein the substrate is insulated metalsubstrate, the insulated metal substrate comprising: a base layer; adielectric layer formed on the base layer; and a circuit layer formed onthe dielectric layer, a portion of the circuit layer forming the atleast one conductive mounting pad.
 3. The substrate of claim 1, whereinthe at least one solder flow stop is non-wetting, during a solder reflowprocess, to the solder used for mounting the at least one die.
 4. Thesubstrate of claim 1, wherein the at least one solder flow stop is madeof aluminum.
 5. The substrate of claim 1, wherein the at least onesolder flow stop is aluminum and has a bond pad area extendingtherefrom.
 6. The substrate of claim 5, wherein the solder used is madeof tin-lead.
 7. The substrate of claim 2, wherein the at least onesolder flow stop surrounds at least a portion of the circuit layer. 8.The substrate of claim 2, wherein the at least one solder flow stop hasan upper surface level with the upper surface of the circuit layer. 9.The substrate of claim 1, wherein the at least one solder flow stop hasan upper surface that is elevated above the upper surface of the atleast one conductive mounting pad.
 10. The substrate of claim 2, whereinthe at least one solder flow stop abuts the circuit layer directly,there being no gap between the at least one solder flow stop and thecircuit layer.
 11. The substrate of claim 2, wherein there is a gapbetween the at least one solder flow stop and the circuit layer.
 12. Thesubstrate of claim 1, wherein the solder used is an alloy selected fromthe group of alloys consisting of tin-lead, tin-lead-silver,tin-antimony, lead-indium, tin-lead-indium, tin-bismuth, andtin-lead-bismuth.
 13. The substrate of claim 1, wherein the substrate isselected from the group of substrates consisting of epoxy-glasssubstrates, paper phenolic substrate, direct-bond copper, ceramicsubstrates, silicon substrates, printed circuit boards, printed wiringboards, and flexible circuits.
 14. A process for forming a substrate,with at least one conductive mounting pad and at least one solder flowstop, comprising the steps of: forming the substrate; forming the atleast one conductive mounting pad in or on the substrate; and formingeach of the at least one solder flow stop surrounding at least a portionof one conductive mounting pad, at least one die being soldered to eachof the at least one conductive mounting pad, and wherein each of the atleast one solder flow stop restrains solder from flowing beyond thesolder flow stop during solder reflow, thereby preventing misalignmentof the at least one die during solder reflow.
 15. The process of claim14, wherein the substrate is an insulated metal substrate.
 16. Theprocess of claim 14, wherein the at least one solder flow stop is formedof aluminum.
 17. The process of claim 14, wherein the at least onesolder flow stop is formed with an upper surface that is elevated abovethe upper surface of the at least one conductive mounting pad.